Choose the word which is different from the rest.

The answer is D. Rhombus.

A rectangle is a quadrilateral with four sides and four right angles. A square is a special type of rectangle with all four sides of equal length. A triangle is a polygon with three sides and three angles. A rhombus is a quadrilateral with four sides of equal length, but its angles do not have to be right angles.

In other words, a rhombus is the only shape in the list that is not a rectangle or a square.